THE EYE 



383 



z^ ^Lachrymal 



The Lachrymal Apparatus. Seeing requires that the 

 light penetrate to the retina. For this reason all the 

 structures in front of the retina are transparent. One of 

 these structures, the cornea, on account of its exposure to 

 the air, is liable to become dry, like the skin, and to lose its 

 transparency. To preserve the transparency of the cornea, 

 and also to lubricate the eyelids and 

 aid in the removal of foreign bodies, 

 a secretion, called tears, is constantly 

 supplied. 



The lachrymal, or tear, glands are 

 situated at the upper and outer 

 margins of the orbits. They have 

 the general structure of the salivary 

 glands and discharge their liquid by 



small ducts beneath the upper lids. Fia r6 3-~ Diagram of 

 T^ , ,, irrigating system of the 



From here the tears spread over the eye . Af f er w y etting the eye . 



Surfaces of the eyeballs and find ball the tears may also mois- 



their way in each eye to two small ten the air entering the 

 canals whose openings may be seen lungs ' 

 on the edges of the lids near the inner corner (Fig. 163). 

 These canals unite to form the nasal duct, which conveys 

 the tears to the nasal cavity on the same side of the nose. 

 When by evaporation the eyeball becomes too dry, the 

 lids close reflexively and spread a fresh layer of tears over 

 the surface. Any excess is passed into the nostrils, where 

 it aids in moistening the air entering the lungs. 



HYGIENE OF THE EYE 



Defects in Focusing. The delicacy and complexity of 

 the sense organs of sight render them liable to a number 

 of imperfections, or defects, the most frequent and impor- 

 tant being those of focusing. Such defects not only result 



